Southern Ocean mechanisms of glacial CO₂ drawdown and their links with global climate

Abstract

The Southern Ocean is a major player in Earth’s climate, regulating carbon exchange between the deep ocean and the atmosphere. However, its role under altered climate states is uncertain. In particular, the Southern Ocean is thought to have driven oceanic uptake of >800 Pg of carbon from the atmosphere and other sources during glacial times, providing potential insight into how this important sink for anthropogenic carbon might evolve in future. However, producing unequivocal evidence of such carbon storage and unravelling the interplay of processes driving it pose an ongoing challenge. This thesis, through both proxy data and numerical modelling experiments, makes significant contributions towards both these efforts.

Here, new Southern Ocean proxy data document a strong inverse correlation between atmospheric CO₂ and deep ocean carbon content over the Last Glacial Cycle, providing the clearest evidence to date of ocean carbon storage driving CO₂ drawdown. Quantifying total carbon storage in these waters accounts for a large part of glacial CO₂ change. Next, two modelling studies explore novel mechanisms of CO₂ change linking the high northern and southern latitudes. The first finds, in a series of Earth system models, that enhanced North Pacific ventilation (evidenced by glacial proxy data) directly reduces the carbon and nutrient load of waters upwelling in the Southern Ocean, decreasing CO₂ outgassing and improving biological pump efficiency. The second explores the thermal impacts of northern ice sheets in an idealised ocean-sea ice model, finding that maintaining cool northern temperatures (while elsewhere warms) sustains a “glacial-like” ocean state, promoting ocean carbon retention and curtailing atmospheric CO₂ rise by ~73%.

This work provides a new interhemispheric perspective on glacial CO₂ change, highlighting remote drivers of Southern Ocean processes as an important focus for future research and the interplay between different regions as critical in understanding the ocean carbon cycle in a changing climate.